Faraday s Law Faraday Stuff 10 20 14 9 18 AM Charge carriers experience magnetic forces because they are moving with the wire magnetic force On section two the charge carriers move along the wire due to the There must be some electric potential difference between the ends of the wires because charges are moving o Lets try the above again but well change our frame of reference to that of the wire sitting on the wire The source of the electric field is due to the change of magnetic flux phi o The negative sign comes about because EMF opposes the change of flux Direction of induced EMF can be found using Lenz s Law Direction of induced EMF always opposes the change in flux induced EMF will only oppose a CHANGE in flux not the flux itself Example Example o Area penetrated by B ext is increasing o Flux phi is increasing o Induced EMF wants to oppose the increase of net flus o Induced field B ind fights B ext direction is opposite to decrease the net magnetic flux phi o B ind points down which implies that induced current is clockwise curl fingers in direction of current then thumb is in the direction of magnetic field o EMF is as shown with positive terminal on the top o NOTE the entire analysis is valid if the magnetic field is moving and the wire is staying still as long as the magnetic field is moving to the right in the above case Other methods of inducing charge in flux Changing the angle between loop normal and B ext Changing the magnitude of magnetic field Applications of Faraday s Law Important Points Strictly speaking Faraday s Law only applies to closed circuits o Faraday s Law o Can NOT find flux through an open wire not in a closed loop using Faraday s Law can be handled by using expression for induced motional Electric field If the wire is moving only inside the magnetic field No net induced EMF because there is no change in flux o o Magnetic field confined to a small area inside the loop and the magnetic field never penetrates the conducting wire o o Example Resistive Forces due to Faraday Current 10 20 14 9 18 AM Resistive Force How much external work is required to keep the loop moving at a constant velocity o Where is all this energy going o The power is dissipated through the internal resistance in the loop because it has its own internal resistance so the wire heats up o o Note Eddy currents sometimes generated through this phenomenon Eddy currents Solid conducting material moving inside a magnetic field or in a changing magnetic field Induced Electric Field Real field that mediates the change in magnetic field to the circuit as an electric field o Therefore changing the magnetic field generates an electric field Above equation does not depend upon whether we have an actual circuit Circuit conducting path of wire Loop just a drawing of an imaginary area Example o Note the electric field causes a current to run clockwise so a magnetic field is created to oppose the magnetic field on the solenoid I which is counter clockwise o
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